In case you want to edit core file. (Make sure you set your page visible to anybody).
if (qa_is_logged_in()){
$qa_content['custom'] = $custompage['content'];
}
else {
return 'Please register to see this content.' ;
}
This will affect every custom page. If you want to set the view rules to a single specific page, you need its pageid, too. The code will be more complicated. So, again, I would suggest you create a PAGE module plugin.